The invention relates to a process for the production of a more particularly steel metal strip having portions of differential thickness by rolling for tailored blanks to be cut to length.
To use material economically and dimension structural members in dependence on loading, for a fair number of years structural members have been used which have a differential thickness and/or material composition over their surface and/or length (xe2x80x9cUmformtechnikxe2x80x9d=Shaping Technology, 7th Aachen Steel Colloquium of Mar. 26-27, 1992, xe2x80x9c4.2 Rolling of loading-optimised longitudinal sectionsxe2x80x9d by B. Hachmann, R. Kopp, Aachen). The rough products of such structural members are known as tailored blanks. In practice basically two different manufacturing procedures are used for the production of tailored blanks.
In a first production process, widely adopted in practice, a start is made from two sheets of different thickness and/or different material composition which are butt welded to one another. The advantage of such a production process is that inexpensive technical equipment is needed for its performance. However, since the welding process will not permit high production speeds this kind of production is suitable only for making s mall numbers of t h e product. Another disadvantage is that at the weld seam there are abrupt transitions from one sheet to another. The result is that strong and abrupt increases in stress occur under loading. Tailored blanks having optimum loading-dependent dimensions over their whole surface cannot therefore be obtained by such a process.
In another prior art process, not yet significantly adopted in practice, a strip is cold rolled in portions with different roll nips. Such a process is known as xe2x80x9cflexible rollingxe2x80x9d (xe2x80x9cFlexibly rolled sheets for loading-adapted workpiecesxe2x80x9d by Schwarz, Kopp, Ebert and Hauger in the Journal xe2x80x9cWerkstatt und Betriebxe2x80x9d=xe2x80x9cWorkplace and Factoryxe2x80x9d 131 (1998) 5). In contrast with the first-mentioned process, this process enables gentle transitions from a thicker band portion to a thinner one and vice versa to be achieved by changing the roll nip. For this reason tailored blanks produced from such a strip can be more satisfactorily loading-optimised than tailored blanks obtained from butt welded sheets of differential thickness. However, one disadvantage of this process is that the reduction of the roll nip calls for extremely high rolling forces. Investigations of an actual rolling problem showed that if the roll nip is adjusted by 1.5 mm, the rolling force must be increased by 23000 kN. With an estimated roll stand rigidity of approximately 6000 kN per mm, the result is a spring travel of 3.8 mm, by which the roll stand expands. A hydraulic adjustment of the roll nip must therefore be able to take up both the 3.8 mm expansion and also the 1.5 mm difference in thickness. This requires a very elaborate technical installation and is therefore also very expensive. Another disadvantage of flexible rolling for the production of tailored blanks to be cut to length is that, due to the extremely high rolling forces, short transitions in thickness of, for example, 20 mm in length can be produced only if the rolling speed is very low. It was discovered that with the aforementioned rolling forces it is possible to achieve adjusting speeds of approximately 10 mm/sec, something which gives an adjusting time of 0.53 sec with the aforementioned adjusting travels. A rolling speed of 40 m/min then gives a ramp-shaped transition from the thicker portion to the thinner portion of approximately 353 mm in length. Such a long ramp is clearly too long for the majority of applications of tailored blanks. Consequently, by this process of flexible rolling it is completely impossible to obtain higher rolling speeds of more than 100 m/min if short transition pieces are required.
In a completely different prior art rolling process (DE-PS 505 468) which relates not to the production of tailored blanks to be cut to length from a strip, but to the production of a smooth strip, the strip is first rolled between rolls, at least one of which has a generated surface corrugated in the peripheral direction. A strip corrugated on one or both sides transversely of the strip longitudinal direction produced in this way is then rolled smooth. The meaning and purpose of corrugated rolling is to roll strips using rollers of relatively small diameter without subsequent roll sagging.
It is an object of the invention to provide a rolling process for tailored blanks to be cut to length from the strip and an apparatus suited to said process; the process/apparatus enable short and gently increasing transitions to be produced between strip portions of differential thickness at high rolling speeds, without the need to use extremely high rolling forces.
This problem is solved in a process of the kind specified by the feature that the portions of the strip of differential thickness are produced by hot rolling, the strip being adjusted in portions to a differential temperature by cooling or heating prior to the hot rolling pass. The apparatus according to the invention suited to the performance of the process is characterised by a rolling mill having hydraulic adjustment means which can be adjusted to a constant rolling force, and a strip cooling or heating device disposed upstream of the rolling mill.
The invention makes use of the physical property of a metal strip that its yield stress depends on temperature. The controlled raising or lowering of strip temperature in certain portions thereof produces portions of differential thickness with a substantially constant rolling force. The temperature level of the strip at which rolling is performed is in any case so high that the strip can be rolled with a rolling force substantially reduced in comparison with cold rolling. The reduced rolling force then also makes possible short transitions from portions of thicker strip to portions of thinner strip and vice versa. Since the rolling forces as a whole are comparatively low, another result is substantially lower values for the upward springing of the roll stand, so that there is also no need for correspondingly expensive compensating devices. Whether the strip to be rolled is either heated or cooled to reduce the yield stress of the metal depends on a number of parameters, more particularly the temperature of the metal strip and its quality, rolling conditions and the grain size of the metal strip to be rolled. Preferably the cooling/heating of the metal strip is performed in a temperature range having as high a temperature-dependent yield stress gradient as possible, since small changes in temperature, which can therefore be achieved with a short treatment time, lead to relatively large changes in the yield stress and therefore also to relatively large changes in strip thickness. In individual cases it is possible to depart from this principle, namely if the level of yield stress is substantially lower with a somewhat lower yield stress gradient. In many steels of both steel qualities the largest possible temperature-dependent yield stress gradient lies between the xcex1 and xcex3 ranges. The difference between the higher strength and soft steels is that in the case of the higher strength steels the xcex3/xcex1 transformation takes place over a larger interval of temperature than in the case of soft steels. With few exceptions, in the case of higher strength steels the temperature interval for the transformation is xcex94Ar=Ar3xe2x88x92Ar1 greater than 150 K, while the temperature interval for the transformation in the case of soft steels is xcex94Ar=Ar3xe2x88x92Ar1 less than 130 K. Alongside this or in addition higher strength and soft steels can also be distinguished by means of the carbon equivalent according to Yurioka (Australian Weld. Res. Ass. Melbourne Mar. 19,20, 1981, Paper 10, pp. 1-18) using the following the formula:
xe2x80x83CEN=C+[0.75+0.25*tan h(20+(Cxe2x88x920.12))]*{Si/24+Mn/6+Cu/15+Ni/20+(Cr+Mo+V+Nb+Ti)/5+5*B}
wherein the alloying contents must be allowed for in percent by weight. The limit value CEN for soft and higher strength steels is CEN=0.1. Higher strength steels lie above this limit value, while soft steels lie therebelow. Lastly, it is also characteristic of hard steel qualities that as a rule they have a carbon content of  greater than 0.0.5 percent by weight, while such content is lower in the case of soft steels.
Due to the aforedescribed differential temperature-dependent yield stress gradient, in the case of soft steels the strip should be rolled in both the austenite and the ferrite zones, namely the band portions thicker after rolling in the austenite zone with the higher temperature, and the cooled strip portions thinner after rolling in the ferrite zone. This differential manner of treatment in the case of soft steels is the result of the course of yield stress, with local minimum and maximum with increasing temperature between the ferrite and austenite zones. The result of this special course of yield stress in the case of soft steels is that there is a smaller strip thickness in the portions cooled prior to hot rolling than in the uncooled portions. With soft steels it is preferred to perform rolling exclusively in the austenite and/or ferrite zone, and not in the transitional zone (xcex1+xcex3). In the case of higher strength steels with sluggish transformation behaviour and relatively small yield stress gradients in the xcex3/xcex1 zone, use can also be made of the biphase zone (xcex3+xcex1), since no abrupt transformation of structure or changing yield stress occurs with slight changes of temperature. In the case of such steel qualities the starting point of the cooling of the metal strip should lie in the biphase zone and the end point of the heating of the metal strip in the biphase zone. At the same time, the hot rolling is preferably performed in the biphase zone (xcex3+xcex1) in those portions of the metal strip whose thickness is to be reduced, and in the ferrite zone (xcex1) in the other portions.
Basically the strip is rolled with a substantially constantly rolling force. However, if provision is made for changing the rolling force, influence can additionally be exerted on the degree of thickness reduction. This is advantageous if the required thickness reduction cannot be achieved solely via a change in temperature. In the hot rolling pass the thickness reduction of the metal strip should also be at least 10% also in the thicker portions.
Suitable materials for the performance of the process are hot strip reheated from the cold state; hot strip maintained at a predetermined temperature in the coil; and hot strip obtained directly from the heat of the melt by a double roller and supplied by the double roller directly to the production process according to the invention. Although after hot rolling the shaping properties of the strip are not as different in the portions of reduced thickness as in a cold rolled strip with strengthening by cold working, after hot rolling the strip can be given a heat treatment to even out the different shaping properties. Thus, the cooled metal strip can be heated to a temperature of more than 600xc2x0 C. However, the strip is preferably maintained at a temperature of over 600xc2x0 C. For temperature maintenance use can be made of the temperature which the strip has after hot rolling. This is advantageously done in the coil. However, a heat treatment can also be performed in an annealing furnace or in hot galvanisation using a temperature of above 600xc2x0 C.
In contrast, if the different material properties obtained by hot rolling are to be maintained in the individual portions, the metal strip can after the rolling pass be uniformly cooled to a temperature xe2x89xa6500xc2x0 C.
To change over the roll nip from thinner band portions to thicker band portions and vice versa without heavy expenditure for technical apparatus, in one embodiment of the invention the constant rolling force is transmitted from the hydraulic adjusting means via a spring assembly to the rolling mill. In that case the springs act after the fashion of force/energy accumulators which react rapidly to the changing resistance to deformation of the strip, without the need to activate the hydraulic adjusting means.
To give the strip differential flow stress in portions via its temperature, use can be made of heating devices, such as inductive heating devices, and also cooling devices. The simplest means of effecting this is that the strip cooling device has nozzles which are directed at the strip surface and operated with compressed air, water or steam. Since the time during which the nozzles act on the strip is one of the decisive factors for the degree of cooling, their action time can be influenced in the direction of lengthening with a high strip speed via the feature that the nozzles can be pivoted or rotated in the direction in which the strip runs. Another possibility is that the nozzle arrangement is entrained parallel with the strip at the same speed.
To ensure that the minimum strip thickness is not exceeded, according to the invention the rolling mill can be adjusted to a given minimum roll nip.